Field of the Invention
The present invention relates to a sheet cooling apparatus used for cooling a sheet, which is formed from paper fibers or the like, that is a target for heating, pressing, and fixing an unfixed toner image in an image forming apparatus such as a copying machine, a printer, or a facsimile machine.
Description of the Related Art
Conventionally, in image forming apparatuses such as electro-photographic apparatuses and electrostatic recording apparatuses, an image is formed on a sheet by forming a toner image on the sheet as a recording material and fixing the toner image using a fixing device by heating and pressing the toner image. As fixing devices used in such image forming apparatuses, a roller fixing system is employed in which a pressing nip portion (fixing nip portion) is formed by pressing a pressure roller to a fixing roller having an internal heater and performing fixing by performing heating and pressing in the pressing nip portion.
In a fixing device of the roller fixing system, heat is applied to toner and a sheet, and accordingly, moisture contained in the sheet evaporates in the pressing nip portion and after passing the pressing nip portion. Then, ripples and curls occur due to a change in the amount of moisture of the sheet and stress applied to the sheet.
When a sheet is viewed in the level of fibers, the sheet is formed by entangling short fibers with each other, moisture is contained inside the fibers or between the fibers, and the fibers and water generate hydrogen bonding. In a fixing process, when heat is applied to the sheet, the moisture included inside the sheet evaporates, and accordingly, the hydrogen bonding occurs between the fibers, whereby the sheet is deformed. When the sheet is left around, the sheet absorbs moisture from the environment, and the hydrogen bonding between the fibers is broken again. However, moisture is not permeated between some fibers, whereby the deformation is maintained.
As a pattern of the deformation, there are deformations according to a difference in the expansion and contraction between the front and rear sides of a sheet and deformation according to a difference in the expansion and contraction between a center portion and an end portion of a sheet. In accordance with such deformation, ripples and curls occur in the sheet.
In order to solve such a problem, a configuration is disclosed in which the sheet is cooled as below.
In Japanese Patent Laid-Open No. 2009-161347, a bending portion bending in a direction opposite to the bending of a sheet on which an image is fixed by a fixing portion in the conveying direction is included. In addition, a cooling member that cools a sheet conveyed by a belt member through the belt member in an area including the bending portion and a pressing member that presses a sheet to the bending portion side of the cooling member are included.
In Japanese Patent Laid-Open No. 2009-175260, a configuration is disclosed in which an endless belt member having good heat conductivity is stretched over belt cooling rollers that are aligned from a fixing device in the conveying direction. Then, a sheet heated by the fixing device is brought into contact with the endless belt member stretched between the belt rollers so as to be cooled, and the endless belt member heated by the sheet is cooled by the belt cooling rollers.
However, among the above-described conventional technologies, in the technology disclosed in Japanese Patent Laid-Open No. 2009-161347, there is concern that tension of the belt member may increase, and deterioration due to abrasion caused by a contact load of the belt member for the cooling member may become serious.
In addition, in the technology disclosed in Japanese Patent Laid-Open No. 2009-175260, since the contact area between the endless belt member and the belt cooling rollers is small, there is concern that the heat transfer to the endless belt member is lowered in accordance with an increase in the number of continuously passing sheets.